Composite gas separation modules are commonly used to selectively separate specific gases from gas mixtures. These composite gas separation modules can be made of a variety of materials, but the two most commonly used materials are polymers and metallic composites. While polymer membranes can provide an effective and cost-efficient option for the separation of gases at low temperatures, they are often unsuitable for gas separation processes that require higher temperatures and pressures; because, they tend to thermally decompose. The demand for high-temperature processing, along with tighter environmental regulations, requires composite gas separation modules that provide high flux, high selectivity, and the ability to operate at elevated temperatures.
The prior art discloses various types of and methods for making gas separation membranes that are supported upon porous substrates and that may be used in high temperature gas separation applications. Many of the known techniques for depositing thin, dense, gas-selective membrane layers onto porous substrates use techniques that often leave a surface that is not uniform in thickness. One the techniques developed to produce a membrane having a more uniform thickness described in U.S. Pat. No. 7,390,536. This patent discloses a method for fabricating a composite gas separation module by depositing a first material on a porous substrate forming a coated substrate, which may be abraded or polished to remove unfavorable morphologies from its surface forming a polished substrate. Thereafter, a gas-selective metal such as palladium or a palladium alloy can be deposited to form a dense gas-selective membrane over the porous substrate. However, there is no suggestion that such abrading or polishing may be used for the purpose of providing enhanced activation properties to the surface of a membrane layer so that chemical activation or activation by seeding the surface of a with the nuclei of a hydrogen-selective material is not required. In fact, U.S. Pat. No. 7,390,536 expressly discloses that after polishing, the surface of the polished porous substrate should be chemically activated prior to depositing a subsequent layer of a gas-selective metal.
Another method for fabricating a palladium composite gas separation module is disclosed in U.S. Patent Publication No. 2009/0120287, which presents a method of making a metallic composite gas separation membrane system. The membrane system can comprise a porous support, a first membrane layer of a gas-selective material overlying the porous support where a substantial portion of the membrane layer is removed by the use of an ultra-fine abrasive to reduce the membrane thickness, and a second layer of a gas-selective material overlaying the reduced membrane layer. The first membrane layer may comprise palladium that is deposited by multiple plating cycles. This palladium membrane layer is then abraded to remove a substantial portion of the membrane to reduce its thickness and polished to a smoother finish. A second palladium layer is subsequently deposited onto the newly reduced layer. The abrading step provides for a reduction in the membrane thickness, but there is no mention of it providing for a special surface morphology having enhanced activation properties for the placement or deposition thereon of an additional metal membrane layer.
In many of the prior art methods of making metal membranes for use in gas separation that are supported upon a porous substrate, the surface of the porous substrate and the surfaces of the metal layers and membranes between each application thereof are required to be surface activated by contacting them with an activation solution. An example of such an activation solution includes a mixture of stannous chloride (SnCl2), palladium chloride (PdCl2), hydrochloric acid (HCl), and water. This method of activation often requires multiple applications of the activation solution with intervening drying and, even, annealing. These wash and dry steps are laborious, they produce hazardous aqueous wastes, and they require a substantial amount of time to complete.
Another method of activation of a palladium surface utilizes palladium acetate in chloroform solution and involves evaporation, drying and decomposition of the acetate followed by reduction to palladium metal seeds.
A non-chemical method for activating the surface of metals is disclosed in U.S. 2011/0232821. However, the disclosed method employs a different surface morphology, in particular, a different surface roughness, than employed in the present inventive method.
Thus, it is desirable to have a method of making a supported metal membrane that is thin, dense and relatively uniform in thickness that may be used in the separation of gases.
It is further desirable for the method to allow for multiple metal plating steps in the manufacture of a supported metal membrane without the need for intermediate chemical activation of the surfaces of the support and of the intermediate metal membrane layers.
It is also desirable for the method to generate reduced amounts of waste products and volatile organic solvents in the manufacturing of a supported metal membrane.